It is often desirable to have a liquid-proof barrier for containing a large body of liquid or liquid-solid material. For example, waste landfills and liquid impoundments are two types of impoundments in which the fill volume may contain toxic liquids. Some sort of barrier is necessary to prevent leakage that would contaminate the environment.
Geomembrane liners are one type of barrier used for containing such waste materials. These liners are made from large sheets of flexible material such as plastic, which is typically a high-density polyethylene or other polymeric material. In addition to being flexible, the liner material is selected to resist deterioration as a result of contact with the waste.
The liner is constructed to define the bottom and sides of the impoundment and to form an impermeable barrier. Generally, to form the liner, sheets of the material are seamed together at the site to form one or more continuous layers.
Often, to provide an additional margin of containment, the liner is comprised of two separate layers of liner material. The methods described above must then be adapted if leaks in the upper liner are to be detected. An inter-liner zone between the liners has been used with such methods.
A persistent problem with both single layer and double layer liners is their susceptibility to leaks, such as punctures and seam leaks. These leaks can occur as a result of installation and operational factors. When waste materials are contained within the impoundment, such leaks can contaminate the environment if not remedied. Thus, it is necssary to know whether a leak is present, and if so, where the leak is, so that remedial action can be taken.
When a new liner is installed, locating and repairing leaks is relatively simple. Conventional methods include visual inspection, vacuum box testing, mechanical testing, arc discharge testing, and air lance testing. All of these methods require the impoundment to be empty and the liner to be exposed.
If the impoundment is filled with material, the ability to accurately locate a leak is critical to the ability to repair it. In other words, repair is much facilitated if the location of the leak is known. Thus, various methods have been developed to locate leaks in liners after they are in use.
Recently, a electrical current detection method has been developed, which requires an electrically conductive medium, such as water, covering the liner. This method is described in the patents listed in the "Related Patents" section above. Generally, a detector is moved through the impoundment to detect the magnetic or electrical effects of current flowing through a leak. Although this method is useful when the liner contains a relatively homogeneous fluid, its usefulness is limited when the material in the impoundment does not have such characteristics, such as when the impoundment contains solid waste.
If the waste in the impoundment is not a relatively homogenous liquid, two methods for locating leaks are a parallel wire dielectric method and a low resistance measuring method. Both involve placing wires under the liner.
The parallel wire dielectric method involves placing parallel wires under the liner during construction. Leaking liquids penetrate the soil between the wires, thus forming a lossy dielectric. The leak is located using time-domain reflectometry, whereby a portion of an electromagnetic pulse propagated along two adjacent wires is reflected and detected. The location and severity of the leak can be determined from time and amplitude measurements of the reflected pulse.
The low resistance measuring method uses electrical wires arranged in a grid under the liner. The media under the liner separates the grid wires from each other. Leaks are indicated by detecting a decrease in resistance between two or more wires. The leak is located by locating the intersection of the two perpendicular wires having low resistance.
A limitation of both wire methods is that they are affected by fluctuations in the groundwater level. Also, contaminant liquids may remain in the sub-liner media after the leak is repaired, making the method less useful for detecting subsequent leaks. Another disadvantage of the resistance measuring method is that the wires must be bare, so that corrosion and chemical action may destroy the wires.
Another method that could be used for locating leaks in a solid-filled impoundment is disclosed in U.S. Pat. No. 4,725,785. This patent describes use of the electrical current detection method generally described above, but also discusses a method for locating leaks in the lower liner of a two-layer geomembrane liner, where the potential difference in the middle area between the liners is inaccessible to direct measurement. The area between the liners is filled with liquid. A voltage measuring detector rotates near the perimeter of the impoundment and measures potential differences. An anomaly in potential difference indicates a leak and by measuring potential difference as a function of the detector's azimuthal position, the leak can be located.